code: def test_selelct_all_variables_when_variables_is_none(df_na):
      imputer = DropMissingData(missing_only=False, variables=None)
      X_transformed = imputer.fit_transform(df_na)
      assert imputer.n_features_in_ == 6
      assert imputer.variables_ == [
          "Name",
          "City",
          "Studies",
          "Age",
          "Marks",
          "dob",
      ]
      assert X_transformed.shape == (5, 6)
      assert X_transformed[imputer.variables_].isna().sum().sum() == 0
  
file_path: feature_engine/tests/test_imputation/test_drop_missing_data.py
imports: import pytest
  from feature_engine.imputation import DropMissingData
label_desc: Use image node_id 2 for calling a global function with example usage: test_selelct_all_variables_when_variables_is_none(df_na) without return types